Data can be stored in the form of information layers of optical record carriers such as compact discs (CDs), conventional digital versatile discs (DVDs) and so-called Blu-Ray™ discs.
Blu-Ray™ discs have recently been proposed following the advent of blue laser diodes that emit light at a significantly shorter wavelength than the red laser diodes used to read or write data from conventional DVDs. As the wavelength of the blue laser diode is shorter than that of more commonly used red laser diodes, the blue laser diode can form a smaller spot on the disc, and hence the information layer tracks of Blu-Ray™ discs can be more closely spaced than those of conventional DVDs, which in turn means that Blu-Ray™ discs can have a greater storage capacity than conventional DVDs—typically at least a twofold increase in storage capacity can be obtained.
To avoid customers having to purchase a variety of different devices for reading or writing data from or to specific types of optical record carrier, it is desirable for a single optical scanning device to be capable of reproducing data from a number of optical record carriers of different formats.
However, this aim is not easy to accomplish as the different record carrier formats and the associated scanning devices have differing characteristics. For example, CDs are available, inter alia, as CD-A (CD-audio), CD-ROM (CD-read only memory) and CD-R (CD-recordable), and are designed to be scanned with a laser wavelength of about 785 nm and a numerical aperture (NA) of 0.45. DVDs, on the other hand, are designed to be scanned at a laser wavelength in the region of 650 nm, and Blu-Ray™ discs are designed to be scanned at a laser wavelength in the region of 405 nm. For reading DVDs and NA of 0.6 is generally used, whereas for writing DVDs and NA of 0.65 is generally required.
A complicating factor is that discs designed to be read out at a certain wavelength are not always readable at another wavelength. An example is the CD-R in which special dyes are applied in the recording stack in order to obtain a high signal modulation at 785 nm wavelength. At 650 nm wavelength the modulation of the signal from the disc becomes so small due to the wavelength sensitivity of the dye that readout at this wavelength is not feasible.
When introducing a new record carrier system with higher data capacities it is important that the new devices for reading and writing are backward compatible with the existing record carriers in order to obtain a high acceptance level in the market. Therefore, the DVD system must contain a 785 nm laser and a 650 nm laser to be able to read all existing CD types. Similarly, a system capable of reading all of CD, DVD and Blu-Ray™ discs should contain a 785 nm laser, a 650 nm laser and a 405 nm laser.
Different types of record carrier also differ in the thickness of their transparent substrates, which typically act as a protective layer for the data carrying layer of the disc, and as a result the depth of the data layer from the entrance face of the record carrier varies from record carrier type to record carrier type. For example, the data layer depth for DVDs is about 0.6 mm, whereas the data layer depth for CDs is about 1.2 mm. The spherical aberration incurred by the radiation beam traversing the protective layer is generally compensated in an objective lens of the optical scanning device.
As a result of these different characteristics for different types of record carrier, problems can result if it is attempted to read data, for example, from a record carrier with an optical scanning device that has been optimized for another, different type of record carrier. For example, large amounts of spherical aberration and a non-negligible amount of spherochromatism can be caused if one type of carrier medium is read with an objective lens that has been optimized for another. The device could be provided with three objective lenses, one for each wavelength. However, this solution would be relatively expensive.
It is therefore highly preferable to provide a device which has a single optical objective lens for scanning a variety of different optical carrier mediums using different wavelengths of laser radiation.
International patent application WO 02/082437 describes such an objective lens for use within an optical scanning device for reading data from three different types of record carrier. The lens has a phase structure which is arranged in the path of the radiation beam. This phase structure comprises a plurality of phase elements of different heights which when viewed in profile are arranged as a series of steps. The different heights of the phase elements are related and arranged so as to produce a desired wavefront modification of the radiation beam of a specific wavelength for reading a specific type of record carrier.
Systems of the type described by WO 02/082437 provide a solution to the problem of scanning three different types of optical record carrier with the associated different wavelengths of radiation beam using one objective lens within the optical scanning device. However, the phase structures involved are often of a complex nature, the phase elements having a large range of different heights. Such phase structures can be difficult to design and manufacture to a level at which a high optical efficiency for each wavelength is achieved. Additionally they can be relatively expensive to manufacture.
Various systems have been proposed in which a fluid system is used to provide an optical element with variable characteristics.
U.S. Pat. No. 5,973,852 describes a fluid filled variable power optical lens. The lens includes a housing having an optically transparent elastic membrane disposed over one end of a chamber which contains a fluid. A pump assembly is used for inserting or withdrawing fluid from the chamber, whereby the membrane is correspondingly selectively bulged outwardly or inwardly in the shape of a convex or concave lens. International patent application WO 00/58763 describes an electrowetting based system whereby the curvature of a fluid meniscus between two different fluid bodies can be changed. It is proposed that the system may be used as a variable lens.
U.S. Pat. No. 6,288,846 describes systems in which a fluid system can be switched between two different discrete states in order to provide different wavefront modifications. A refractive index difference of approximately zero is established between a fluid and a wavefront modifier, when the system is in one of these states, in order to leave the radiation beam unchanged. In the other state of the system this refractive index difference is of a sufficient value such that the path of the radiation beam is modified. A fluid-handling system is used to switch the fluid system. Examples of the fluid handling system include manually or motorized hypodermic syringes, peristaltic pumps, compressible bulbs, and piezoelectric, hydraulic or pneumatic actuators.
U.S. Pat. No. 6,408,112 describes an optical switch which includes a fluid system housed in channels and cavities within the component. Piezoelectric actuators, which are arranged in the cavities, cause the liquid to be displaced in the channels. In one embodiment, the liquid passes over the face of a wavefront modifier including a relief structure in the form of a Fresnel lens. In one embodiment, one of the two fluid components of the fluid system is a gas which is compressed when the liquid is moved into place over the relief structure. However, such a component requires an upright orientation to be maintained in order to prevent the gas from being located at the part of the system containing the piezoelectric pump. It is also described that two suitable liquids may be used. However, one drawback is the reliability of the switching process, particularly when a relief structure is used, where fluid flow is not entirely smooth. Requiring smooth fluid flow during switching also limits the speed of switching.